Method And Device For Operating A Fermentation Plant

ABSTRACT

A fermentation plant with at least one digester ( 1 ) is supplied with a substrate ( 1   a ) that is percolated using a percolate ( 2   a ) from a percolate container ( 2 ) and biogas that is generated via the percolation process in the digester ( 1 ) or/and in the percolate container ( 2 ) is drawn off. The drawn off biogas is treated in a biogas treatment device ( 3 ) and CO 2 -containing exhaust gas that is generated during the treatment is introduced into the substrate ( 1   a ) in the digester ( 1 ) using a purge line ( 4 ), for forcing out the biogas in the substrate ( 1   a ). CO 2 -containing biogas that was generated in an additional digester ( 1 ) in a different fermentation process can be introduced into the substrate ( 1   a ) in the digester ( 1 ) using a purge line ( 4 ). The CO 2 -containing purge gas supplied to the substrate ( 1   a ) is forced to pass through the substrate ( 1   a ) dislodging any trapped biogas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/948,147, titled “Method and Device For Operating A FermentationPlant” filed on Nov. 17, 2010 and incorporated fully herein byreference.

TECHNICAL FIELD

The invention relates to a method for operating a fermentation plant andmore particularly, to a method and device that separates biogas from CO₂gas and returns the CO₂ gas to the digester of the fermentation plant,forcing the CO₂ gas to pass through the substrate in the digester toforce out and recover any trapped biogas located in the substrate.

BACKGROUND INFORMATION

In typical fermentation methods, a percolate flows through a substratein a fermentation digester and is collected primarily at the bottom ofthe digester, returned to a percolate container and from there can befed to the digester again. This creates methane-containing biogas in thedigester and also in the percolate container, said biogas beingdischarged from the digester and/or the percolate container and fed to abiogas treatment system to increase the methane concentration. Withbiogas treatment, the biogas to be obtained is separated from otherundesired components and is supplied for further use. Among otherthings, the treatment produces a significant amount of CO₂ gas.

To supply the biogas at the end of the process from the digester to thebiogas treatment system, the biogas-containing volume above thesubstrate must be purged after the fermentation process using purge gasin order to eliminate the biogas from the volume. EP 1 997 875 A1discloses the use of CO₂-containing gas from the treatment system forthis purpose.

The fermentation process also generates a considerable amount of biogas,which is, however, “trapped” in the substrate itself. This is releasedduring substance handling or reprocessing of the digistate. Thus, thereleased biogas portions, in particular methane, are not recovered andenter uncontrolled into the atmosphere.

SUMMARY

It is, therefore, one object of the invention to provide a method thatdoes not have the aforementioned disadvantages and that reduces themethane loss to a minimum.

The present invention features a method for operating a fermentationplant with at least one digester (1) to which a substrate to be digested(1 a) is supplied, whereby the substrate (1 a) is percolated with apercolate (2 a) from a percolate container (2), and biogas that isgenerated in the digester (1) or/and in the percolate container (2) iswithdrawn, whereby the withdrawn biogas is treated in a biogas treatmentdevice (3) and wherein CO2-containing exhaust gas that is generatedduring treatment in the treatment device (3) is introduced into thedigester (1) using a purge line (4) for removing any biogas that wasgenerated in the digester (1), whereby the CO2-containing purge gas isintroduced into the digester (1) such that it passes through thesubstrate (1 a). In one embodiment, the CO₂-containing purge gas isintroduced into the substrate (1 a) such that any gas that is present inthe substrate (1 a) is dislodged into the substrate-free volume (1 b) ofthe digester (1). The CO₂-containing purge gas may be introduced into abottom region of the digester (1) and in addition, such that theCO₂-containing purge gas passes through the substrate (1 a) essentiallyuniformly from bottom to top.

After purging with the CO₂-containing purge gas, the present method mayalso provide fresh air that is introduced into the digester (1) via thepurge line (4). During the supply of the CO₂-containing purge gas, thebiogas is withdrawn via a withdrawal line (12, 23). After the conclusionof the purging process with CO₂-containing gas, a biogas withdrawal portis closed and a venting line (11) is opened.

The present invention also discloses and features a device for carryingout the method as set forth above. The device includes at least onedigester (1) for supplying a substrate (1 a), a percolate container (2)that is connected to a digester (1) via a percolate return line (21 b)and a percolate supply line (21 a) for receiving percolate (2 a), atleast one biogas withdrawal line (12, 23) for removing biogas that hasbeen generated in the digester (1), with a purge line (4) forintroducing purge gas into the digester (1), whereby a treatment device(23) for treating biogas and/or at least one additional digester (1) isprovided, whereby the treatment device (3) for the purpose of supplyingCO₂-containing exhaust gas generated during the treatment process via anexhaust gas line (31) and/or the additional digester (1) for supplyingthe CO₂-containing biogas that was generated during the fermentationprocess in the additional digester (1) can be connected with the purgeline (4), whereby a port on the digester (1) for the purge line (4) isarranged such that during the purging process, CO₂-containing gas flowsthrough the substrate that is located in the digester (1).

Accordingly, utilizing the method according to the invention, theCO₂-containing purge gas passes through the substrate. Preferably, thepurge gas is supplied to the digester over a large area at the bottom ofthe digester. In this manner, the substrate is purged and any trappedbiogas contained therein is forced into the volume above the substrateand can be removed to the biogas treatment system in a known manner. TheCO₂-containing purge gas can be obtained as exhaust gas from the biogastreatment system or alternatively and/or as a supplement, it is alsopossible to supply CO₂-containing biogas that was generated in adifferent digester or by a different fermentation process in an earlierphase of the digestion process to the digester that is to be purged.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will bebetter understood by reading the following detailed description, takentogether with the drawings wherein:

FIG. 1 is block diagram of a representative system on which the methodof the present invention can be practiced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A substrate 1 a is supplied to a digester 1. Shown is only one digesterbut systems with numerous digesters are possible. The digester 1 isconnected to a percolate container 2 via a supply line 21 a that is usedto feed percolate 2 a into the digester 1. During the fermentationprocess, the percolate percolates through the substrate 1 a. Thepercolate is collected in the lower region of the digester 1 and isreturned to the percolate container 2 via a return line 21 b.

Biogas is generated during the fermentation process and collects in thedigester primarily in volume 1 b above the substrate 1 a. In addition,biogas is present in a trapped manner in the substrate 1 a and cannoteasily enter into the volume 1 b. Furthermore, additional biogas isgenerated in percolate container 2. The biogas that is present in theregions 1 b and 1 a is removed by the purge process according to theinvention. For this purpose, a biogas line 12 is provided betweendigester 1 and percolate container 2 and an additional line 23 betweenpercolate container 2 and a treatment device 3. The CO₂-containing gasthat is generated during the treatment process in the treatment device 3is used for purging. Thus, CO₂-containing gas refers to a gas thatcontains predominantly CO₂. It may contain CO₂ exclusively but othercomponents are conceivable.

The biogas treatment generates—in addition to highly concentratedmethane, which is provided via line 32—predominantly CO₂-containingexhaust gas that can be removed via Line 33 or can be introduced vialine 31 into the purge line 4 that leads to the digester 1. The purgeport at the digester 1 is positioned such that the purge gas that isintroduced via purge line 4 passes through the substrate. Preferably,the port for the purge line 4 is provided at the bottom 1 c or in thebottom area of the digester 1 or on the side in a region 1 d of thedigester 1 that is still covered by the substrate 1 a. In addition, adistribution device (not shown) can be provided to guide the purge gasthrough the substrate at several locations. This can be accomplished inthe digester 1 or through splitting the purge line 4 before the digester1. In any case, several purge gas ports can be provided for introducingthe purge gas into the substrate 1 a of the digester 1.

The CO₂-containing purge gas is supplied under pressure to the digester1 such that any biogas that is still trapped in the substrate 1 a isdislodged from the substrate 1 a. The port to the biogas line 12 or acorresponding valve 12 a, respectively, is opened such the biogas thathas been dislodged from the substrate 1 a and the substrate-free volume1 b can be removed via the percolate container 2 and the line 23 to thetreatment device 2. Once the biogas has been removed from the digester1, or a specified switchover point is reached that can be, for example,tied to a threshold value of the biogas (generally methane gas)concentration, the valve 12 a in line 12 can be closed again and theexhaust valve 11 a to the exhaust line 11 can be opened; the valve 31 aof the CO₂-line will be closed and the CO₂-supply will be stopped. Nowan additional purge is carried out using fresh air that is provided tothe purge line 4 via a fresh air line 5. The exhaust gas that escapesthrough the exhaust line 11 is subjected to an exhaust gas treatmentand/or discharge that shall not be described in detail here.

In the shown processing variant, an exhaust gas from the biomethanetreatment is used as the purge gas. Biogas that is generated in anotherdigester (not shown) can be used in place of the exhaust gas. A largeamount of CO₂ is generated in the biogas in the first phase of thefermentation process. This CO₂-containing biogas from a digester can beused for purging another digester, for example by connecting the purgegas port of the digester to be purged to the biogas line of anotherdigester.

Using the method according to the invention, the methane gas loss can bereduced significantly by the method according to the invention. Inaddition, the efficiency of the biogas production can be increasedbecause the biogas that is present in the substrate can be dislodged andsupplied to the treatment system as well.

Modifications and substitutions by one of ordinary skill in the art areconsidered to be within the scope of the present invention, which is notto be limited except by the following claims.

1-7. (canceled)
 8. A digester device, comprising: at least one digester(1), configured for treating a substrate (1 a) with percolate (2 a) andwith a CO₂-containing purge gas; a percolate container (2), fluidlycoupled to said at least one digester (1) via a percolate return line(21 b) and a percolate supply line (21 a), said percolate container (2)configured for providing said percolate (2 a) to said at least onedigester (1), at least one biogas withdrawal line (12, 23), configuredfor removing biogas that has been generated in at least one of thedigester (1) and the percolate container (2) during a substrate (1 a)fermentation period, and for providing said removed biogas to a biogastreatment device (3); a biogas treatment device (3), configured fortreating biogas, whereby the biogas treatment device (3) is configuredfor supplying CO₂-containing exhaust gas generated during the treatmentof biogas to said substrate (1 a) in said digester (1) via an exhaustgas line (31) fluidly coupled to a purge line (4), said purge line (4)configured for introducing CO₂-containing purge gas into the digester(1), for removing any biogas in said substrate (1 a) that was generatedin the digester (1) during e fermentation period, whereby theCO₂-containing purge gas is introduced into the digester (1) such thatsaid CO₂-containing purge gas passes through the substrate (1 a) in thedigester (1), wherein CO₂-containing purge gas is introduced Underpressure into a bottom region of the digester (1) and this into a bottomregion of the substrate (1 a) through one or more purge ports such thatany biogas that is present in the substrate (1 a) is dislodged into asubstrate-free volume (1 b) of the digester (1) above the substrate (1a) for subsequent collection and removal.
 9. (canceled)
 10. The digesterof claim 8, wherein the CO₂-containing purge gas is distributed into thebottom of the digester (1) such that the CO₂-containing purge gas passesthrough the substrate (1 a) essentially uniformly from bottom to top.11. The digester of claim 8, wherein said biogas is generated in boththe digester (1) and in the percolate container (2).
 12. The digester ofclaim 8, wherein said digester is configured to first provide saidCO₂-containing purge gas into the bottom of the digester (1) via saidpurge line (4) and subsequently provide fresh air into the digester (1)via said purge line (4).
 13. the digester of claim 8, wherein saiddigester is configured after the conclusion of a purging process withCO₂-containing gas, to cause a biogas withdrawal port (12 a) to closewhile a venting line Port (11 a) to open.